1. Field of the Invention
The present invention relates to a proton conductive solid polymer electrolyte to be used for an electrochemical cell including, for example, a fuel cell and a hydrogen and oxygen generator for generating hydrogen and oxygen by electrolyzing water.
2. Description of the Related Art
In the fuel cell, for example, an electrolyte is interposed between an anode to which fuel gas containing hydrogen is supplied and a cathode to which oxygen-containing gas such as air is supplied. In the electrolyte, hydrogen ion (proton), which is generated by ionizing hydrogen contained in the fuel gas on the anode, is moved to the cathode. In other words, the electrolyte serves as a proton conductor in the fuel cell.
A material obtained by humidifying a perfluorosulfonic acid polymer membrane with liquid water is widely known as an example of the proton conductor which acts as the electrolyte of the fuel cell. However, the proton conductivity of the membrane is lowered as the membrane is dried. Accordingly, in order to maintain the power generation characteristics of the fuel cell, the membrane is prevented from being dried by adding steam to the fuel gas and/or the oxygen-containing gas to continuously supply the membrane with water, and supplying a cooling medium into the fuel cell to maintain the operation temperature at 80 through 90° C.
Recently, a composite electrolyte comprising a base material of basic solid polymer such as polybenzimidazole is proposed. The base material is doped or impregnated with inorganic strong acid liquid such as phosphoric acid (see U.S. Pat. No. 5,525,436 and Japanese Laid-Open Patent Publication No. 11-503262 (PCT)). Another composite electrolyte comprises a base material of meta-polyaniline and is prepared in the same manner as described above (see Japanese Laid-Open Patent Publication No. 2001-160407).
The two types of the composite electrolytes described above have high proton conductivities even in a dried state. Therefore, it is unnecessary to use any humidifier. Further, it is sufficient to use a small-scale cooling system, because the fuel cell can be operated at high temperature. Therefore, the fuel cell system can be structured simply, and it is possible to realize a small size. As described above, it is advantageous to adopt the electrolyte which is excellent in proton conductivity. Accordingly, it has been vigorously tried to develop electrolytes which are excellent in proton conductivity.
For example, K. D. Kreuer et al. reported that a material, which is obtained by adding imidazole to polyetherketone sulfonic acid as an acidic polymer, has a proton conductivity of about 0.01 S/cm at 160° C. in a dried state, in J. Maier. Electrochimica (1998), Vol. 43, pp. 1281-1288.
C. Yang et al. reported that a material, which is obtained by adding imidazole to Nafion as an acidic polymer (trade name of perfluorosulfonic acid polymer membrane produced by DuPont), exhibits a proton conductivity of 0.08 to 0.09 S/cm at 160° C. in a dried state, in J. Power Sources (2001), Vol. 103, pp. 1-9.
Japanese Laid-Open Patent Publication No. 2001-236973 discloses a solid polymer electrolyte comprising a main polymer which is acidic or basic and a subsidiary polymer which is capable of forming an acid/base composite structure together with the main polymer. The subsidiary polymer is disposed into portions of the main polymer other than acidic or basic portions thereof. For example, it is described that a material, which is obtained by doping or impregnating Nafion with polyethyleneimine, exhibits a value of 1.7×10−3S/cm at 150° C.
It is sincerely desired to obtain an electrolyte having a proton conductivity which is much more excellent that those of the electrolytes as described above.